R. E. Majors (High-performance Liquid Chromatography, Advances and Perspectives, Vol. 1, C. Horvath, ed., Academic Press, 1980, pp. 80-84) describes the commonly used monolayer, monofunctional bonded-phase silicas. These bonded-phase silicas are employed in chromatographic applications and do not utilize sterically-protecting groups to create materials that are stable to a wide range of operating conditions. U.S. Pat. Nos. 3,722,181 and 3,795,313 (J. J. Kirkland and P. C. Yates) deal with products and processes for chromatographic packings with polymeric stationary phases prepared from bi- or tri-functional silanes, and do not involve surfaces with sterically-protected monofunctional silanes. German Pat. No. DE 2,605,789 (J. H. Knox and A. Pryde) describe bonded-phase materials for liquid chromatography prepared by reacting silica supports with trimethyl- or triethyl-silane groups: no use of special sterically-protecting groups was made. The concept of utilizing sterically-protecting groups was also not employed by Lindgren et al. (J. Chromatogr., Volume 296 (1984) 83-95), for materials prepared from single or mixed monofunctional silanes, including triethylchlorosilane. K. K. Unger et al. J. Chromatogr., Volume 125 (1976) 115-127 describe the synthesis of bonded-phase materials for chromatography, including those containing diphenylsilyl-, triphenylsilyl-, and n-butyldiphenylsilyl- groups. The planar phenyl groups do not provide the steric protection required to produce highly stable, effective chromatographic packings.
The reaction of t-butylchlorodimethylsilane with silica gel to form a chromatographic packing was reported by G. E. Berendsen and L. de Galan (J. Liq. Chromatogr., 1 (1978) 561). These workers found very low retention and poor selectivity with the dimethyl-t-butyl bonded phase, which they attributed to the very low coverage with this functionality and a high concentration of residual SiOH groups on the silica surface after reaction. Glass beads also were reacted with t-butylchlorodimethylsilane (C. A. Ku, J. D. Henry, R. Siriwardane and L. Roberts, J. Colloid Interface Sci., 106 (1985) 377-387). In both of these studies, silanes with groups other than dimethyl on the silicon atom were not described. Substituted t-butyl protecting groups also were not disclosed.
Co-pending application Ser. No. 06/936,084 discloses the use of monofunctional silanes containing at least two sterically-protecting groups attached to the silicon atom of the silane. Compositions with a single protecting group were not disclosed.
A general review of organosilicon reagents for protecting certain functional groups of soluble compounds in organic synthesis is given by Lalonde et al. [Synthesis, Volume 9, (1985) 817]. Corey et al [Tetrahedron Letters, Volume 22, 3455 (1981)] describe the triisopropyl-silyl hydroxyl protecting group as providing strong steric screening for the silicon atom to which it is attached, as well as the atom attached to the silicon. This is in contrast to trimethylsilyl or triethylsilyl protecting groups. The triisopropylsilyl group was not used with any solid substrates. The reagent, t-butylpentafluoro-phenylmethyl-chlororsilane was used for the formation of hydrolytically stable, alkylsilyl derivatives by Poole et al. [J. Chromatogr., Volume 187 (1980) 331, 40]. No solid substrates were employed, and in this case only one sterically-protecting group was present on the silane reagent. U.S. Pat. No. 4,478,952 (M. P. Mack and C. T. Berge) describes Ti(OSi(isopropyl).sub.3).sub.4 as soluble, supported catalysts for olefin polymerization; no covalent bonding to a substrate was involved.
Szabo et al. [Helvetica Chimica Acta, Volume 67 (1984) 2128] describe the grafting of dimethylneopentylsilane to the surface of fumed silica. The use of a single neopentyl group does not afford the steric protection required to achieve a highly stable packing. The use of silane reactions with silica in which R.sub.3 SiCl was employed, where R =methyl, ethyl, propyl (NOT isopropyl), was described by Drago et al. [Inorg. Chem., Volume 24 (1985) 1983]. No sterically-protecting groups of the type used in this invention were utilized. None of the above references disclose materials or processes that are a feature of this invention.
The silane bonded phases described to date suffer from the disadvantage of being hydrolytically unstable in aggressive mobile phases such as those containing trifluoroacetic acid at pH&lt;3. This results in decreased column lifetime as well as excessive "bleeding" of the bonded phase into the mobile phase and gives less than pure components from the separation. The support structures of this invention have bonded phases with improved stability.